This invention relates to a method of investigating the electrical resistivity of the ground wherein an electrical current is passed through the ground between a first pair of positions (called herein "current electrode positions") and a measurement is made of the resulting potential difference between a further pair of positions (called herein "potential electrode positions"). Such a method is hereinafter called "an electrical resistivity method". The invention also relates to apparatus for use in the method.
In one known electrical resistivity method, the potential electrode positions lie on a straight line between the current electrode positions and the three spaces between adjacent electrode positions are all equal. This is known as the Wenner electrode arrangement. A series of determinations of electrical resistivity are made with the same electrode arrangement but with different electrode spacings. The values of electrical resistivity which are found are used to plot a curve showing the relation between apparent resistivity and electrode spacing. This curve can be interpreted in terms of horizontal sub-surface strata of different resistivities and this method is one example of an electrical sounding method. Electrode arrangements other than the Wenner electrode arrangement are used in other electrical sounding methods.
Measurements made by the known electrical resistivity method hereinbefore referred to can be affected by the presence of local resistivity variations. By "local" I mean herein variations which are significant only in a region near to the electrodes. The distance across the region in which the variations is significant is not greater than twice the distance between the electrodes which are furthest apart.
One disadvantage of the known method hereinbefore described is that the potential difference measured between a particular pair of potential electrode positions can be affected by a local resistivity variation which arises from the presence in the vicinity of the electrode positions of a sub-surface mass of material which has electrical properties different from those of the stratum containing the mass. The relationship between the electrode spacing and the apparent resistivity which is calculated from such an affected potential difference measurement will not be repesentative of the strata under investigation and will mislead the investigator.
The effect on the potential difference measured may be a positive one, by which I mean that the apparent resistivity calculated from the affected measurement is greater than the apparent resistivity which would be found if the stratum in which the mass occurs is substantially homogeneous. Alternatively, the effect on the potential difference measured may be a negative one, by which I mean that the apparent resistivity calculated is smaller than the apparent resistivity which would be found if the stratum was substantially homogeneous. Whether the effect is a positive one or a negative one depends both upon the electrical properties of the mass and upon the position of the mass relative to the electrodes. In the vicinity of the electrodes there is a plurality of zones, five zones in the case of the Wenner electrode arrangement, and a mass which gives rise to a positive effect on the potential difference in one zone will give rise to a negative effect on the potential difference if the mass is in an adjacent zone.